A target detection device is known example of detecting a target by using a wave such as a sound wave, a radio wave, or a light wave, as in a sonar system, a radar system, or a lidar system. The system uses pulse transmission in which a transmitter transmits a wave, and after a lapse of predetermined time, transmits a wave again. Pulse transmission clarifies at which time point the wave corresponding to a reflected wave from the target is transmitted, and further, can remove influence due to scattering from a medium near the transmitter.
In a case of continuously detecting a target, the transmitter continuously transmits a wave. In this case, in order to determine at which time point the wave corresponding to a reflected wave from the target is transmitted, the transmitter modulates and transmits a wave. In order to reduce influence of scattering from a medium near the transmitter, it is necessary to install a receiver and the transmitter at a predetermined distance apart, as described in PTL 1, or to change a beam shape or a direction of a beam of the transmitter or the receiver by using adaptive beamforming.
However, when the transmitter and the receiver cannot be installed at a predetermined distance apart, influence of the scattering cannot be reduced sufficiently. In some cases, changing a beam shape does not cause to sufficiently reduce influence of the scattering. Further, in a case of changing a direction of a beam, a reflected wave from a target becomes small, and a target cannot be detected.
PTL 2 describes, as a method of avoiding influence of scattering from a medium, a method of separating from the scattering from the medium a reflected wave from a target by using Doppler shifting of a frequency of the reflected wave from the target in a case where the target is moving. PTL 3 describes that a covering state of a ground is recognized based on a difference between two scattered waves produced by reflection of two radio waves having different polarization properties.
Besides, the PTL 4 describes, as a technology of avoiding influence of a scattered wave, a technology of irradiating a ground with a microwave to find correlation between a backward scattering coefficient of a radar image and a vegetation ground. PTL 5 describes a technology of changing transmission electric power of a modulated pulse signal, depending on a distance to a target, in order to avoid a reception signal failure due to a modulated pulse signal. PTL 6 describes a technology of separating a scattered wave from a mixed wave of a progressive wave advancing toward a target in a medium and a forward scattered wave scattered forward from the target, to search for the target. PTL 7 describes a technology of changing reception sensitivity, depending on delay time of reception, to detect a target by using an underground radar.